High strength welding joint having excellent impact toughness at very low temperature, and flux-cored arc welding wire therefor

ABSTRACT

A high strength welding joint having excellent toughness at low temperature obtained by welding a cryogenic high-strength high-Mn steel, comprising 0.1-0.61 wt % of C, 0.23-1.0 wt % of Si, 14-35 wt % of Mn, 6 wt % or less of Cr, 1.45-3.5 wt % of Mo, 0.02 wt % or less of S, 0.02 wt % or less of P, 0.001-0.01 wt % of B, 0.001-0.2 wt % of Ti, 0.001-0.3 wt % of N, and balance of Fe and inevitable impurities; and a flux-cored arc welding wire comprising 0.15-0.8 wt % of C, 0.2-1.2 wt % of Si, 15-34 wt % of Mn, 6 wt % or less of Cr, 1.5-4 wt % of Mo, 0.02 wt % or less of S, 0.02 wt % or less of P, 0.01 wt % or less of B, 0.1-0.5 wt % of Ti, 0.001-0.3 wt % of N, 4-15 wt % of TiO 2 , 0.01-9 wt % of at least one of SiO 2 , ZrO 2  and Al 2 O 3 , 0.5-1.7 wt % of at least one of alkali elements including K, Na, and Li, 0.2-1.5 wt % of at least one of F and Ca, and balance of Fe and inevitable impurities.

TECHNICAL FIELD

The present disclosure relates to a high-strength welding joint having ahigh degree of cryogenic impact toughness and a flux-cored arc weldingwire for forming the high-strength welding joint, and more particularly,to a welding joint having a high degree of low-temperature impacttoughness and a high degree of room-temperature yield strength due tohigh-toughness austenite being maintained in the welding joint evenunder cryogenic environments, and a flux-cored arc welding wire forforming the welding joint.

BACKGROUND ART

Along with the recent explosive increase in the demand for liquefiednatural gas (LNG), the demand for transportation facilities and storagetanks for transporting and storing LNG has also explosively increased.Thus, tanks for transporting or storing LNG are required to have astructure capable of resisting impacts at temperatures equal to or lowerthan the temperature of LNG, −162° C. For this, materials having a highdegree of cryogenic impact toughness such as aluminum (Al), 9% Ni steel,and stainless steel (hereinafter referred to as STS) are generally used.

However, aluminum (Al) has a low degree of tensile strength, and thusthick aluminum plates have to be used. In addition, aluminum (Al) haspoor weldability. Moreover, welding materials formed of 9% Ni steel (forexample, Inconel 625 having a nickel content of 50 wt % or greater and achromium content of 20 wt % or greater) are expensive and have a lowdegree of weld zone yield strength. STS has problems such as high pricesor low thermal deformation, and the cryogenic properties of STS have notbeen guaranteed.

Therefore, what is needed is the development of cryogenic high-manganesewelding joints including manganese (Mn) instead of relatively expensivenickel (Ni) to guarantee weldability and stabilize austenite.

To this end, the formation of welding joints having an impact toughnessof 27 J or greater may be required to secure the stability of weldstructures at a very low temperature, −196° C. or less. The grade ofyield strength of high-manganese (Mn) steels was 360 MPa at roomtemperature according to earlier studies. However, the grade of yieldstrength of the current high-manganese steels is 500 MPa to 800 MPa.Therefore, if welding materials having a degree of yield strength of 360MPa are used, welding joints having relatively low strength are formed.When such structures including welding joints having relatively lowstrength are designed, the design may be focused on the welding joints,and thus thick steel sheets may have to be used.

Therefore, to address these problems, welding materials having a degreeof room-temperature yield strength within the range of 400 MPa orgreater are required. In the related art, due to this reason, weldingmaterials having high contents of nickel (Ni) and chromium (Cr) (Ni: 50wt % or greater, and Cr: 20 wt % or greater) are used to guarantee therequired strength. That is, in the related art, welding materials orwelding joints having low contents of alloying elements and incurringlow costs have not been developed.

DISCLOSURE Technical Problem

An aspect of the present disclosure may provide a welding joint formedby submerged, flux-cored, or gas metal arc welding and having a highdegree of low-temperature impact toughness and a high degree ofroom-temperature yield strength due to high-toughness austenite beingmaintained in the welding joint even under cryogenic environments, andhigh-temperature cracking is prevented during welding.

An aspect of the present disclosure may also provide a flux-cored arcwelding wire for forming the welding joint.

However, aspects of the present disclosure are not limited thereto.Additional aspects will be set forth in part in the description whichfollows, and will be apparent from the description to those of ordinaryskill in the related art.

Technical Solution

According to an aspect of the present disclosure, there is provided ahigh-strength welding joint having a high degree of cryogenic toughnessand obtained by welding parts of a cryogenic high-strengthhigh-manganese steel, the high-strength welding joint including, by wt%, carbon (C): 0.1% to 0.61%, silicon (Si): 0.23% to 1.0%, manganese(Mn): 14% to 35%, chromium (Cr): 6% or less, molybdenum (Mo): 1.45% to3.5%, sulfur (S): 0.02% or less, phosphorus (P): 0.02% or less, boron(B): 0.001% to 0.01%, titanium (Ti): 0.001% to 0.2%, nitrogen (N):0.001% to 0.3%, and a balance of iron (Fe) and inevitable impurities.

The high-strength welding joint may further include at least oneselected from tungsten (W), niobium (Nb), and vanadium (V) in a totalamount of 5 wt % or less.

The high-strength welding joint may further include yttrium (Y) and/or arare earth metal (REM) in an amount of 0.1 wt % or less.

The high-strength welding joint may further include nickel (Ni) in anamount of 10 wt % or less.

The high-manganese steel may have a basic composition ofMn₂₄C_(0.4)Cr₄Si_(0.3).

According to another aspect of the present disclosure, there is provideda flux-cored arc welding wire having a high degree of strength and ahigh degree of cryogenic impact toughness, the flux-cored arc weldingwire including, wt %, carbon (C): 0.15% to 0.8%, silicon (Si): 0.2% to1.2%, manganese (Mn): 15% to 34%, chromium (Cr): 6% or less, molybdenum(Mo): 1.5% to 4%, sulfur (S): 0.02% or less, phosphorus (P): 0.02% orless, boron (B): 0.01% or less, titanium (Ti): 0.09% to 0.5%, nitrogen(N): 0.001% to 0.3%, TiO₂: 4% to 15%, at least one selected from SiO₂,ZrO₂, and Al₂O₃: 0.01% to 9% in total content, at least one selectedfrom potassium (K), sodium (Na), and lithium (Li): 0.5% to 1.7% in totalcontent, at least one of fluorine (F) and calcium (Ca): 0.2% to 1.5%,and a balance of iron (Fe) and inevitable impurities.

The flux-cored arc welding wire may further include at least oneselected from tungsten (W), niobium (Nb), and vanadium (V) in a totalamount of 5 wt % or less.

The flux-cored arc welding wire may further include yttrium (Y) and/or arare earth metal (REM) in an amount of 1 wt % or less.

The flux-cored arc welding wire may further include nickel (Ni) in anamount of 10 wt % or less.

Advantageous Effects

The welding joint according to the aspect of the present disclosure hasa high degree of low-temperature impact toughness and a high degree ofroom-temperature yield strength because high-toughness austenite ismaintained in the welding joint even under cryogenic environments andhigh-temperature cracking is prevented during welding. Therefore, thewelding joint may be usefully applied to cryogenic containers such asliquefied natural gas (LNG) tanks.

In addition, the flux-cored arc welding wire according to the aspect ofthe present disclosure may be effectively used to form the welding jointhaving a high degree of low-temperature impact toughness and a highdegree of room-temperature yield strength, and thus a welding structurehaving a high degree of impact toughness in cryogenic environments, forexample, −196° C. or lower, may be formed using the flux-cored arcwelding wire.

BEST MODE

Hereinafter, technical ideas of the present disclosure will be describedaccording to exemplary embodiments.

First, a welding joint having a high degree of cryogenic toughness willbe described according to an exemplary embodiment of the presentdisclosure.

The welding joint of the exemplary embodiment is formed by welding partsof a cryogenic high-strength high-manganese steel, and the welding jointincludes, by wt %, carbon (C): 0.1% to 0.61%, silicon (Si): 0.23% to1.0%, manganese (Mn): 14% to 35%, chromium (Cr): 6% or less, molybdenum(Mo): 1.45% to 3.5%, sulfur (S): 0.02% or less, phosphorus (P): 0.02% orless, boron (B): 0.001% to 0.01%, titanium (Ti): 0.001% to 0.2%,nitrogen (N): 0.001% to 0.3%, and a balance of iron (Fe) and inevitableimpurities. The characteristics of the alloying elements and thecritical significance of the content range of each of the alloyingelements will now be simply described.

Carbon (C): 0.1 wt % to 0.61 wt %

Carbon (C) may be currently the most powerful element effective inguaranteeing the strength of the welding joint and stabilizing austenitethat guarantees the cryogenic impact toughness of the welding joint.Thus, in the exemplary embodiment, the addition of carbon (C) isrequired. It may be sufficient that the lower limit of the content ofcarbon (C) be 0.1 wt %. However, if the content of carbon (C) is greaterthan 0.61 wt %, gases such as carbon dioxide gas may be generated duringa welding process to cause defects in the welding joint, and carbon (C)may combine with alloying elements such as manganese (Mn) or chromium(Cr) and may form carbides such as MC or M₂₃C₆ to cause a decrease inlow-temperature impact toughness. Therefore, it may be preferable thatthe content of carbon (C) be within a range of 0.1 wt % to 0.61 wt %.

Silicon (Si): 0.23 wt % to 1.0 wt %

Silicon (Si) is added to remove oxygen from the welding joint andguarantee the spreadability of welding beads. If the content of silicon(Si) is insufficient (that is, less than 0.23 wt %), the fluidity of thewelding joint may decrease. Conversely, if the content of silicon (Si)is greater than 1.0 wt %, segregation may occur in the welding joint,thereby decreasing the low-temperature impact toughness of the weldingjoint and negatively affecting the weld crack sensitivity of the weldingjoint. Therefore, according to the exemplary embodiment of the presentdisclosure, it may be preferable that the content of silicon (Si) bewithin a range of 0.23 wt % to 1.0 wt %.

Manganese (Mn): 14 wt % to 35 wt %

Manganese (Mn) is an element facilitating the formation of austenitewhich is stable at a low temperature, and thus the addition of manganese(Mn) is required in the exemplary embodiment. Manganese (Mn) isrelatively inexpensive compared to nickel (Ni). If the content ofmanganese (Mn) is less than 14 wt %, austenite is not sufficientlyformed, and thus the cryogenic toughness of the welding joint may bevery low. However, if the content of manganese (Mn) is greater than 35wt %, segregation may occur excessively, and high-temperature crackingmay occur. In addition, toxic fumes may be generated. Therefore, it maybe preferable that the content of manganese (Mn) be within a range of 14wt % to 35 wt %.

Chromium (Cr): 6 wt % or Less

Chromium (Cr) is a ferrite stabilizing element, and if a certain amountof chromium (Cr) is added, the amounts of austenite stabilizing elementsmay be decreased. Although chromium (Cr) is not added, if the contentsof carbon (C) and manganese (Mn) are high, an austenite matrix may bemaintained. Thus, the lower limit of the content of chromium (Cr) iszero. However, if the content of chromium (Cr) is greater than 6 wt %,chromium carbides may be excessively formed, and thus cryogenictoughness may decrease. Therefore, it may be preferable that the contentof chromium (Cr) be within a range of 6 wt % or less.

Molybdenum (Mo): 1.45 wt % to 3.5 wt %

Molybdenum (Mo) may increase the strength of the matrix of the weldingjoint, and if the content of molybdenum (Mo) is greater than 1.45 wt %,the tensile strength of the welding joint may be 400 MPa or greater. Inaddition, molybdenum (Mo) included in austenitic welding materialsnarrows the width of a solid-liquid coexisting region during welding,thereby suppressing high-temperature cracking. However, if the contentof molybdenum (Mo) is greater than 3.5 wt %, molybdenum carbides may beexcessively formed, and thus cryogenic toughness may decrease.Therefore, it may be preferable that the content of molybdenum (Mo) bewithin a range of 1.45 wt % to 3.5 wt %.

Sulfur (S): 0.02 wt % or Less

Sulfur (S) leads to the formation of a complex precipitate, MnS.However, if the content of sulfur (S) is greater than 0.02 wt %,compounds having a low melting point such as FeS are formed, and thushigh-temperature cracking may be caused. Therefore, it may be preferablethat the content of sulfur (S) be within a range of 0.02 wt % or less.

Phosphorus (P): 0.02 wt % or Less

Phosphorus (P) has an effect on low-temperature impact toughness andforms brittle phosphorus compounds along grain boundaries. Therefore,the upper limit of the content of phosphorus (P) may preferably be 0.02wt %.

Boron (B): 0.001 wt % to 0.01 wt %

Boron (B) segregates along grain boundaries. The segregation of boron(B) along grain boundaries improves the strength of the grainboundaries, and thus the strength of the welding joint may be improved.This effect is sufficiently obtained even when the content of boron (B)is 0.001 wt %. If the content of boron (B) is 0.01 wt % or greater,although strength is markedly increased, low-temperature impacttoughness is decreased. Therefore, according to the exemplary embodimentof the present disclosure, it may be preferable that the content ofboron (B) be within a range of 0.001 wt % to 0.01 wt %.

Titanium (Ti): 0.001 wt % to 0.2 wt %

Titanium (Ti) is included in the welding joint in the form of oxides ornitrides. Such oxides or nitrides (or carbonitrides) exist along grainboundaries and function as nuclei forming sites at high temperatureduring solidification, thereby decreasing the size of austenite grains.In addition, such oxides or nitrides (or carbonitrides) included in themicrostructure of the welding joint improve the strength of the weldingjoint. This strength-increasing effect is obtainable even if the contentof titanium (Ti) is 0.001 wt %. Thus, the lower limit of the titanium(Ti) is 0.001 wt % in the exemplary embodiment. However, if a largeamount of titanium (Ti) is added, the impact toughness of the weldingjoint may decrease. For example, if the content of titanium (Ti) isgreater than 0.2 wt %, although the strength of the welding jointmarkedly increases, the low-temperature impact toughness of the weldingjoint may decrease. Therefore, according to the exemplary embodiment ofthe present disclosure, it may be preferable that the content oftitanium (Ti) be within a range of 0.001 wt % to 0.2 wt %.

Nitrogen (N): 0.001 wt % to 0.3 wt %

Nitrogen (N) has the same effect as carbon (C). Nitrogen (N) formsnitrides together with titanium (Ti). Even if the content of nitrogen(N) is 0.001 wt %, nitrogen (N) may improve the strength of the weldingjoint together with titanium (Ti). Therefore, the content of nitrogen(N) may preferably be 0.001 wt % or greater. However, if the content ofnitrogen (N) is greater than 0.3 wt %, voids may easily be formed in thewelding joint, and the cryogenic impact resistance of the welding jointmay decrease because nitrogen (N) increases the formation of nitridestogether with titanium (Ti). Therefore, the upper limit of the contentof nitrogen (N) may preferably be 0.3 wt %.

The above-described alloying elements are basic alloying elements thatmay be included in the welding joint of the exemplary embodiment, andthe following alloying elements may be further included in the weldingjoint so as to further improve the properties of the welding joint.

At Least One Selected from Tungsten (W), Niobium (Nb), and Vanadium (V):5 wt % or Less in Total Content

Tungsten (W), niobium (Nb), or vanadium (V) increases the strength ofthe welding joint at room temperature and may be optionally included inthe welding joint of the exemplary embodiment. These elements combinewith carbon (C) included in the welding joint and form carbides (orcarbonitrides), thereby improving the tensile strength of the weldingjoint at room temperature. However, if the total content of theseelements is greater than 5 wt %, cracks may easily be formed, and thecryogenic impact toughness of the welding joint may decrease. Therefore,the total content of at least one of tungsten (W), niobium (Nb), andvanadium (V) may preferably be set to be 5 wt % or less.

Yttrium (Y) and/or a Rare Earth Metal (REM): 0.1 wt % or Less

Yttrium (Y) and/or a rare earth metal (REM) may be optically included inthe welding joint of the exemplary embodiment. Yttrium (Y) and/or a rareearth metal (REM) may form oxides at a high temperature, and the oxidesmay function as nuclei forming sites at a high temperature duringsolidification, thereby decreasing the size of austenite grains. In thismanner, Yttrium (Y) and/or a rare earth metal (REM) improve the strengthof the welding joint. However, if the content of yttrium (Y) and/or arare earth metal (REM) is greater than 0.1 wt %, defects may be formedin the welding joint during a welding process. Thus, the content ofyttrium (Y) and/or a rare earth metal (REM) may be adjusted to be 0.1 wt% or less. That is, according to the exemplary embodiment, the contentof Yttrium (Y) and/or a rare earth metal (REM) may preferably be 0.1 wt% or less.

Nickel (Ni): 10% or Less

Nickel (Ni) may be optionally included in the welding joint of theexemplary embodiment as an austenite stabilizing element. If nickel (Ni)is added to the welding joint, the low-temperature impact toughness ofthe welding joint is steeply increased because nickel (Ni) increases thestacking fault energy of the welding joint. That is, nickel (Ni)increases the low-temperature impact toughness of the welding joint. Inaddition to decreasing the strength of the welding joint, nickel (Ni)increases the price of welding materials. Therefore, preferably, thecontent of nickel (Ni) may be adjusted to be 10 wt % or less.

The welding joint includes iron (Fe) and inevitable impurities inaddition to the above-described alloying elements. However, the additionof other alloying elements is not precluded.

The welding joint of the exemplary embodiment may be applied to varioushigh-manganese steels required to have high strength and highlow-temperature impact toughness under very low temperature conditions.That is, the welding joint of the exemplary embodiment is not limited touse for base metals having particular compositions. For example, thewelding joint of the exemplary embodiment may be applied to ahigh-manganese steel having Mn₂₄C_(0.4)Cr₄Si_(0.3) as a basiccomposition.

Next, a flux-cored arc welding wire having a high degree of strength anda high degree of impact toughness will be described in detail accordingto an exemplary embodiment of the present disclosure.

The flux-cored arc welding wire of the exemplary embodiment includes, bywt %, carbon (C): 0.15% to 0.8%, silicon (Si): 0.2% to 1.2%, manganese(Mn): 15.0% to 34.0%, chromium (Cr): 6% or less, molybdenum (Mo): 1.5%to 4%, sulfur (S): 0.02% or less, phosphorus (P): 0.02% or less, boron(B): 0.01% or less, titanium (Ti): 0.09% to 0.5%, nitrogen (N): 0.001%to 0.3%, TiO₂: 4% to 15%, at least one selected from SiO₂, ZrO₂, andAl₂O₃: 0.01% to 9% in total content, at least one selected frompotassium (K), sodium (Na), and lithium (Li): 0.5% to 1.7% in totalcontent, at least one of fluorine (F) and calcium (Ca): 0.2% to 1.5%,and the balance of iron (Fe) and inevitable impurities. Thecharacteristics of the alloying elements and the critical significanceof the content range of each of the alloying elements will now be simplydescribed.

Carbon (C): 0.15 wt % to 0.8 wt %

Carbon (C) may be currently the most powerful element effective inguaranteeing the strength of welding joints and stabilizing austenitethat guarantees the cryogenic impact toughness of welding joints. Thus,in the exemplary embodiment, the addition of carbon (C) is required. Ifthe content of carbon (C) is low, austenite is not stabilized, and thusit is required to properly maintain the content of carbon (C). The lowerlimit of the content of carbon (C) is set to be 0.15 wt %. If thecontent of carbon (C) is greater than 0.8 wt %, gases such as carbondioxide gas may be generated during a welding process to cause defectsin welding joints, and carbon (C) may combine with alloying elementssuch as manganese (Mn) or chromium (Cr) and may form carbides such as MCor M₂₃C₆ to cause a decrease in low-temperature impact toughness.Therefore, according to the exemplary embodiment of the presentdisclosure, it may be preferable that the content of carbon (C) bewithin a range of 0.15 wt % to 0.8 wt %.

Silicon (Si): 0.2 wt % to 1.2 wt %

If the content of silicon (Si) is less than 0.2 wt %, oxygen may beinsufficiently removed from welding joints, and the fluidity of weldingjoints may decrease. Conversely, if the content of silicon (Si) isgreater than 1.2 wt %, segregation may occur in welding joints, therebycausing a decrease in low-temperature impact toughness and having anegative effect on weld crack sensitivity. Therefore, according to theexemplary embodiment of the present disclosure, it may be preferablethat the content of silicon (Si) be within a range of 0.2 wt % to 1.2 wt%.

Manganese (Mn): 15.0 wt % to 34.0 wt %

Manganese (Mn) increases work hardening and leads to the formation ofaustenite which is stable at low temperatures. Thus, manganese (Mn) isadded to the wire of the exemplary embodiment. In addition, manganese(Mn) forms carbides together with carbon (C) and functions as anaustenite stabilizing element like nickel (Ni).

If the content of manganese (Mn) is less than 15.0 wt %, austenite isnot sufficiently formed, and thus low-temperature impact toughnessdecreases. Conversely, if the content of manganese (Mn) is greater than34.0 wt %, large amounts of fumes are generated during welding. Thus, itmay be preferable that the content of manganese (Mn) be within the rangeof 15.0 wt % to 34.0 wt %.

Chromium (Cr): 6.0 wt % or Less

Chromium (Cr) is a ferrite stabilizing element, and the addition ofchromium (Cr) enables decreasing the amounts of austenite stabilizingelements. In addition, chromium (Cr) facilitates the formation ofcarbides such as MC or M₂₃C₆. That is, the addition of a certain amountof chromium (Cr) increases precipitation hardening and enablesdecreasing the amounts of austenite stabilizing elements. Therefore, acertain amount of chromium (Cr) may be added. However, the addition ofchromium (Cr) is optional. In addition, since chromium (Cr) is apowerful anti-oxidation element, the addition of chromium (Cr) mayincrease resistance to oxidation in an oxygen atmosphere.

However, if the content of chromium (Cr) is greater than 6.0 wt %, themanufacturing costs of the flux-cored arc welding wire may increase, andthe cryogenic impact toughness of the flux-cored arc welding wire maymarkedly decreases because of precipitation. Therefore, it may bepreferable that a content of chromium (Cr) be within the range of 6.0 wt% or less.

Molybdenum (Mo): 1.5 wt % to 4.0 wt %

Molybdenum (Mo) is an element increasing the strength of welding joints.In the exemplary embodiment, molybdenum (Mo) added to a welding materialin an amount of 1.5 wt % or greater has an effect of increasing thetensile strength of welding joints to 400 MPa or greater. In addition,molybdenum (Mo) included in austenitic welding materials narrows thewidth of a solid-liquid coexisting region during welding, therebysuppressing high-temperature cracking. However, if the content ofmolybdenum (Mo) is greater than 4.0 wt %, molybdenum carbides may beexcessively formed in welding joints, and thus the cryogenic toughnessof the welding joints may decrease. Therefore, it may be preferable thatthe content of molybdenum (Mo) be within a range of 1.5 wt % to 4.0 wt%.

Phosphorus (P): 0.02 wt % or Less

Phosphorous (P) is an impurity causing high-temperature cracking, andthus the content of phosphorus (P) is adjusted to be as low as possible.That is, preferably, the content of phosphorus (P) is adjusted to be0.02 wt % or less so as to prevent high-temperature cracking.

Sulfur (S): 0.02 wt % or Less

Sulfur (S) is an impurity causing high-temperature cracking togetherwith phosphorus (P), and thus the content of phosphorus (P) is adjustedto be as low as possible. If the content of sulfur (S) is greater than0.02 wt %, compounds having a low melting point such as FeS are formed,and thus high-temperature cracking may be induced. Therefore,preferably, the content of sulfur (S) is adjusted to be 0.02 wt % orless so as to prevent high-temperature cracking.

Boron (B): 0.01 wt % or Less

Boron (B) segregates along grain boundaries of welding joints. Thesegregation of boron (B) along grain boundaries improves the strength ofthe grain boundaries, and thus the strength of welding joints may beimproved. Although the content of boron (B) is 0.001 wt %, these effectsare sufficiently obtained. If the content of boron (B) is greater than0.01 wt %, although the strength of welding joints is markedlyincreased, the low-temperature impact toughness of the welding joints isdecreased. Therefore, the upper limit of the content of boron (B) maypreferably be 0.01 wt %.

Titanium (Ti): 0.09 wt % to 0.5 wt %

During welding, titanium (Ti) functions as an arc stabilizer and anoxidizer, thereby making welding joints clean. In addition, afterwelding, titanium (Ti) included in welding joints leads to the formationof oxides and nitrides (or carbonitrides) and thus increases thestrength of the welding joints. Therefore, it may be preferable that thecontent of titanium (Ti) be within the range of 0.09 wt % or greater.However, after welding, titanium (Ti) included in welding joints inlarge amounts decreases the impact toughness of the welding joints. Ifthe content of titanium (Ti) is greater than 0.5 wt %, low-temperatureimpact toughness may decrease although strength markedly increases.Therefore, the upper limit of the content of titanium (Ti) maypreferably be 0.5 wt %.

Nitrogen (N): 0.001 wt % to 0.3 wt %

Nitrogen (N) improves corrosion resistance and stabilizes austenite.That is, the addition of nitrogen (N) leads to an effect similar to theeffect obtainable by the addition of carbon (C). Therefore, nitrogen (N)may be substituted for carbon (C). These effects of nitrogen (N) may beobtained even though a small amount of nitrogen (N) is added. However,if the content of nitrogen (N) is greater than 0.3 wt %, impactresistance markedly decreases. Thus, it may be preferable that thecontent of nitrogen (N) be within a range of 0.001 wt % to 0.3 wt %.

TiO₂ (Titanium Dioxide): 4 wt % to 15 wt %

Titanium dioxide (TiO₂) is a slag former solidifying before a liquidwelding joint solidifies, thereby preventing the liquid welding jointfrom flowing downward and allowing for all-position welding. In theexemplary embodiment, preferably, TiO₂ is added in an amount of 4 wt %or greater so as to obtain the above-described effects. However, if thecontent of TiO₂ is greater than 15 wt %, the contents of oxides inwelding joints may markedly increase, and thus the low-temperatureimpact toughness of the welding joints may decrease. Therefore,according to the exemplary embodiment of the present disclosure, it maybe preferable that the content of TiO₂ be within a range of 4 wt % to 15wt %.

At least one selected from SiO₂, ZrO₂, and Al₂O₃: 0.01 wt % to 9.0 wt %in total content

If the total content of at least one of SiO₂, ZrO₂, and Al₂O₃ is 0.01 wt%, the workability of all-position welding and the quality of weld beadsmay decrease because of poor application and separation of slag andgeneration of unstable arcs. Conversely, if the content of at least oneof SiO₂, ZrO₂, and Al₂O₃ is greater than 9 wt %, the amount andviscosity of molten slag may markedly increase, thereby worseningall-position weldability and bead shapes. In addition, larger amounts ofelements such as silicon (Si) or aluminum (Al) may be included indeposited metal, and thus impact toughness may decrease.

Therefore, it may be preferable that the total content of at least oneof SiO₂, ZrO₂, and Al₂O₃ be within a range of 0.01 wt % to 9 wt %.

At Least One Selected from Potassium (K), Sodium (Na), Lithium (Li): 0.5wt % to 1.7 wt % in Total Content

During welding, these alkali metals may decrease the ionizationpotential of arcs, and thus arcs may be easily generated and stablymaintained. When the total content of these alkali metals is 0.5 wt % orgreater, these effects may be certainly obtained. However, if thecontent of these alkali metals is greater than 1.7 wt %, fumes may beexcessively generated because of high vapor pressure. The alkali metalsmay include one or more of potassium (K), sodium (Na), and lithium (Li).In the exemplary embodiment, effects obtainable by the addition of thealkali metals have no relationship with the content of each of thealkali metals.

Fluorine (F) and/or Calcium (Ca): 0.2 wt % to 1.5 wt %

If the welding wire of the exemplary embodiment further includes calcium(Ca) and fluorine (F) in the form of a compound of fluorine (F) with analkali metal or an alkaline earth metal, effects of the exemplaryembodiment may be improved. If fluoride is added to the welding wire inan amount of 0.2 wt % or greater, fluorine (F) may appear inhigh-temperature arcs and react with hydrogen during welding, therebyeffectively removing diffusion hydrogen by a dehydrogenation reaction.However, if the content of fluoride is greater than 1.5 wt %, fumes maybe excessively generated because of high vapor pressure, and the slagviscosity of a rutile-containing weld pool in which TiO₂ is contained asa main slag component may be excessively decreased to result in theformation of unstable beads. Therefore, it may be preferable that thecontent of fluorine (F) and/or calcium (Ca) be within the range of 0.2wt % to 1.5 wt %.

The above-described alloying elements are basic alloying elements thatmay be included in the welding wire of the exemplary embodiment, and thefollowing alloying elements may be further included in the welding wireso as to further improve the properties of the welding wire.

At Least One Selected from Tungsten (W), Niobium (Nb), and Vanadium (V):5 wt % or Less in Total Content

Tungsten (W), niobium (Nb), and vanadium (V) increase theroom-temperature strength of the welding wire. These elements combinewith carbon (C) included in welding joints and form carbides (orcarbonitrides), thereby improving the tensile strength of the weldingjoints at room temperature. However, if the total content of tungsten(W), niobium (Nb), and vanadium (V) is greater than 5 wt %, cracks mayeasily be formed, and cryogenic impact toughness may decrease.Therefore, according to the exemplary embodiment of the presentdisclosure, it may be preferable that the total content of tungsten (W),niobium (Nb), and vanadium (V) be within a range of 5 wt % or less.

Yttrium (Y) and/or REM: 1 wt % or Less

Yttrium (Y) and a rare earth metal (REM) function as a powerful oxidizerand stabilize arcs during welding. In addition, yttrium (Y) and a rareearth metal (REM) form oxides in welding joints, and the oxides functionas nuclei forming sites at high temperature during solidification,thereby decreasing the size of austenite grains. In this manner, yttrium(Y) and/or a rare earth metal (REM) improve the strength of weldingjoints. However, if the content of yttrium (Y) and/or a rare earth metal(REM) is greater than 1 wt %, defects may be formed in welding jointsduring a welding process. Thus, the content of yttrium (Y) and/or a rareearth metal (REM) may be adjusted to be 1 wt % or less. That is,according to the exemplary embodiment, the content of Yttrium (Y) and/orrare earth metal (REM) may preferably be 1 wt % or less.

Nickel (Ni): 10% or Less

Nickel (Ni) is an element stabilizing austenite. Nickel (Ni) steeplyincreases the low-temperature impact toughness of welding joints byincreasing the stacking fault energy of the welding joints. However,nickel (Ni) decreases strength and increases the price of weldingmaterials. Therefore, preferably, the content of nickel (Ni) may beadjusted to be 10 wt % or less.

The welding wire includes iron (Fe) and inevitable impurities inaddition to the above-described alloying elements. However, the additionof other alloying elements is not precluded.

Hereinafter, the exemplary embodiments of the present disclosure will bedescribed more specifically through examples. However, the followingexamples should be considered in a descriptive sense only and not forpurpose of limitation. The scope of the present invention is defined bythe appended claims, and modifications and variations may reasonablymade therefrom.

BEST MODE FOR INVENTION Example 1

Flux-cored arc welding wires having a diameter of 1.2 mm were prepared.The flux-cored arc welding wires included, by wt %, carbon (C): 0.15% to0.8%, silicon (Si): 0.2% to 1.2%, manganese (Mn): 15% to 34%, chromium(Cr): 6% or less, molybdenum (Mo): 1.5% to 4%, sulfur (S): 0.02% orless, phosphorus (P): 0.02% or less, boron (B): 0.01% or less, titanium(Ti): 0.09% to 0.5%, nitrogen (N): 0.001% to 0.3%, TiO₂: 4% to 15%, atleast one selected from SiO₂, ZrO₂, and Al₂O₃: 0.01% to 9% in totalcontent, at least one selected from potassium (K), sodium (Na), andlithium (Li): 0.5% to 1.7% in total content, at least one of fluorine(F) and calcium (Ca): 0.2% to 1.5%, and a balance of iron (Fe) andinevitable impurities.

Cryogenic high-manganese steel having a basic composition ofMn₂₄C_(0.4)Cr₄Si_(0.3) was used as a base metal, and parts of the basemetal were welded together using the flux-cored arc welding wires. Thewelding process was performed under conditions of a 100% CO₂ protectiongas, about 290 A DC, about 30 V DC, about 31 CPM, and a heat input ofabout 1.7 kJ/mm. In addition, an interlayer temperature was lower than150° C., and a preheating temperature was about 100° C. to remove onlymoisture.

The compositions of welding joints formed through the welding processwere analyzed as illustrated in Table 1 below. In addition, thelow-temperature impact toughness and tensile strength of the weldingjoints were measured according to the compositions of the welding jointsas illustrated in Table 1 below. In detail, a Charpy impact test wasperformed (at −196° C.) to evaluate mechanical properties of the weldingjoints, and results (J) of the test are shown in Table 1 below. Inaddition, the tensile strength (MPa) of the welding joints was measuredas illustrated in Table 1 below.

TABLE 1 Composition of welding joint (wt %) Y + IP C Mn Si Cr P S N Mo BTi Ni V + Nb + W REM (J) RTTS IS1 0.12 28.2 0.65 1.95 0.011 0.007 0.11.67 0.002 0.05 4.7 4.32 — 28 513.4 IS2 0.58 18.1 0.52 1.87 0.013 0.0080.001 1.52 0.001 0.04 — 2.34 — 29 480.9 IS3 0.33 20.1 0.23 0.01 0.0120.008 0.001 1.52 0.004 0.11 5.3 1.23 — 32 448.6 CS1 0.32 18.7 0.52 6.520.014 0.01 0.04 2.03 0.002 0.03 5.1 — — 14 567.7 CS2 0.75 15.2 0.52 0.020.021 0.023 0.001 1.75 0.001 0.01 — — — Cracks CS3 0.58 16.2 1.23 0.030.014 0.005 0.001 1.72 0.001 0.03 — — — Cracks CS4 0.58 18.4 0.42 1.780.012 0.006 0.002 1.65 0.001 0.04 — — Voids IS: Inventive Sample, CS:Comparative Sample, IP: Impact Property, RTTS: Room-Temperature TensileStrength

As illustrated in Table 1 above, in the case of Inventive Samples 1 to 3having compositions proposed in the exemplary embodiment of the presentdisclosure, the welding joints had a degree of low-temperature impacttoughness within the range of 28 J or greater and a degree of tensilestrength within the range of greater than 400 MPa.

However, in the case of Comparative Sample 1 having an excessively highchromium content, the welding joint had a poor degree of low-temperatureimpact toughness, 14 J, even though the tensile strength of the weldingjoint was high. In the case of Comparative Sample 2 having anexcessively high silicon content, cracks were observed in the weldingjoint after the welding process, and in the case of Comparative Sample 3having an excessively high nitrogen content, voids were formed in thewelding joint. In the case of Comparative Sample 4 having a high rareearth element content, voids were formed in the welding joint.

Example 2

Submerged arc welding solid wires having a diameter of 4.0 mm wereprepared. The submerged arc welding solid wires included, by wt %,carbon (C): 0.15% to 0.8%, silicon (Si): 0.5% to 1.5%, manganese (Mn):15% to 32%, chromium (Cr): 5.5% or less, molybdenum (Mo): 1.5% to 3%,sulfur (S): 0.025% or less, phosphorus (P): 0.025% or less, boron (B):0.01% or less, titanium (Ti): 0.05% to 1.2%, nitrogen (N): 0.005% to0.5%, and a balance of iron (Fe) and inevitable impurities. Optionally,the submerged arc welding solid wires included at least one of tungsten(W), niobium (Nb), and vanadium (V): 6 wt % or less in total content,yttrium (Y) and/or a rare earth metal (REM): 1 wt % or less, and nickel(Ni): 10 wt % or less.

Cryogenic high-manganese steel having a basic composition ofMn₂₄C_(0.4)Cr₄Si_(0.3) was used as a base metal, and parts of the basemetal were welded together using the submerged arc welding solid wires.In the welding, an alumina basic flux was used. The welding process wasperformed under conditions of about 600 A DC, about 32 V DC, about 29CPM, and a heat input of about 4.0 kJ/mm. In addition, an interlayertemperature was lower than 150° C., and a preheating temperature wasabout 100° C. to remove only moisture.

The compositions of welding joints formed through the welding processwere analyzed as illustrated in Table 2 below. In addition, thelow-temperature impact toughness and tensile strength of the weldingjoints were measured according to the compositions of the welding jointsas illustrated in Table 2 below. In detail, a Charpy impact test wasperformed (at −196° C.) to evaluate mechanical properties of the weldingjoints, and results (J) of the test are shown in Table 2 below. Inaddition, the tensile strength (MPa) of the welding joints was measuredas illustrated in Table 2 below.

TABLE 2 Composition of welding joint (wt %) Y + IP C Mn Si Cr P S N Mo BTi Ni V + Nb + W REM (J) RTTS IS1 0.1 34.2 0.95 3.23 0.006 0.005 0.263.25 0.01 0.2 9.1 — — 54 477.6 IS2 0.31 25.1 0.65 5.12 0.017 0.009 0.112.12 0.006 0.092 4.3 — — 32 524.9 IS3 0.59 15.2 0.28 0.03 0.012 0.0170.004 1.54 0.001 0.04 — — — 31 427.5 IS4 0.1 28.7 0.62 2.1 0.012 0.0080.09 1.87 0.003 0.07 5.1 4.12 — 27 522.5 IS5 0.52 20.8 0.58 1.73 0.0150.01 0.004 1.54 0.001 0.002 — 2.12 — 29 474.7 IS6 0.35 20.8 0.24 0.050.013 0.007 0.005 1.75 0.005 0.01 5.2 1.1  — 34 437.5 IS7 0.31 19.5 0.481.11 0.014 0.008 0.13 2.05 0.002 0.14 5.3 — 0.005 41 452.9 CS1 0.34 18.90.54 6.73 0.014 0.008 0.013 1.95 0.003 0.001 5.3 — — 16 567.5 CS2 0.5819.4 0.54 1.93 0.011 0.007 0.091 1.78 0.02 0.27 — — — 25 478.2 CS3 0.1831.2 0.46 0.03 0.014 0.007 0.004 1.38 0.002 0.04 9.1 — — 39 396.1 CS40.74 16.9 0.62 0.03 0.025 0.024 0.005 1.67 0.002 0.05 — — — Cracks CS50.62 15.9 1.52 0.01 0.015 0.006 0.003 1.78 0.001 0.04 — — — Cracks CS60.56 15.5 0.66 0.01 0.013 0.008 0.36 1.52 0.002 0.04 — — — Voids CS70.62 18.9 0.49 1.92 0.014 0.008 0.004 1.56 0.001 0.002 — — 0.17  VoidsIS: Inventive Sample, CS: Comparative Sample, IP: Impact Property, RTTS:Room-Temperature Tensile Strength

As illustrated in Table 2 above, in the case of Inventive Samples 1 to 7having compositions proposed in the exemplary embodiment of the presentdisclosure, the welding joints had a degree of low-temperature impacttoughness within the range of 27 J or greater and a degree of tensilestrength within the range of greater than 400 MPa.

However, in the case of Comparative Samples 1 and 2 having a highchromium or boron content, the welding joints had a poor degree oflow-temperature impact toughness, 25 J, even though the tensile strengthof the welding joints was high. In the case of Comparative Sample 3having an excessively low molybdenum content, the tensile strength ofthe welding joint was low, 400 MPa or less, even though thelow-temperature impact toughness of the welding joint was high. In thecase of Comparative Sample 4 having excessively large amounts of carbon(C), phosphorus (P), and sulfur (S), cracks were formed in the weldingjoint, and in the case of Comparative Sample 5 having excessively largeamounts of carbon (C) and silicon (Si), cracks were formed in thewelding joint.

In the case of Comparative Sample 6 having an excessively large amountof nitrogen (N) and Comparative Sample 7 having excessively largeamounts of carbon (C) and a rare earth metal (REM), voids were observedin the welding joints after the welding process.

Example 3

Gas metal arc welding wires having a diameter of 1.2 mm were prepared.The gas metal arc welding wires had the same compositions as thesubmerged arc welding solid wires of Example 2.

Cryogenic high-manganese steel having a basic composition ofMn₂₄C_(0.4)Cr₄Si_(0.3) was used as a base metal, and parts of the basemetal were welded together using the gas metal arc welding wires. Thewelding process was performed under conditions of about 200 A DC, about30 V DC, about 40 CPM, and a heat input of about 0.9 kJ/mm. In addition,an interlayer temperature was lower than 150° C., and a preheatingtemperature was about 100° C. to remove only moisture.

The compositions of welding joints formed through the welding processwere analyzed as illustrated in Table 3 below. In addition, thelow-temperature impact toughness and tensile strength of the weldingjoints were measured according to the compositions of the welding jointsas illustrated in Table 3 below. In detail, a Charpy impact test wasperformed (at −196° C.) to evaluate mechanical properties of the weldingjoints, and results (J) of the test are shown in Table 3 below. Inaddition, the tensile strength (MPa) of the welding joints was measuredas illustrated in Table 3 below.

TABLE 3 Composition of welding joint (wt %) IP C Mn Si Cr P S N Mo B TiNi V + Nb + W Y + REM (J) RTTS IS1 0.11 32.3 0.89 3.25 0.005 0.002 0.232.54 0.009 0.19 8.2 — — 53 476.9 IS2 0.32 29.8 0.62 5.52 0.018 0.01 0.122.23 0.005 0.12 4.5 — — 35 518.4 IS3 0.61 14.2 0.23 0.02 0.013 0.0150.002 1.55 0.002 0.002 — — — 31 424.0 IS4 0.29 19.3 0.45 1.23 0.0120.009 0.12 2.19 0.001 0.13 5.1 — 0.06 36 463.7 CS1 0.57 18.2 0.52 1.870.01 0.007 0.082 1.75 0.019 0.25 — — — 24 480.8 CS2 0.19 32.5 0.47 0.030.012 0.007 0.002 1.4 0.001 0.002 8.2 — — 42 392.1 CS3 0.57 17.2 0.570.02 0.013 0.006 0.35 1.54 0.002 0.002 — — — Voids IS: Inventive Sample,CS: Comparative Sample, IP: Impact Property, RTTS: Room-TemperatureTensile Strength

As illustrated in Table 3 above, in the case of Inventive Samples 1 to 4having compositions proposed in the exemplary embodiment of the presentdisclosure, the welding joints had a degree of low-temperature impacttoughness within a range of 31 J or greater and a degree of tensilestrength within a range of greater than 400 MPa.

However, in the case of Comparative Sample 1 having an excessively highboron content, the welding joint had a poor degree of low-temperatureimpact toughness, 24 J, even though the tensile strength of the weldingjoint was high. In the case of Comparative Sample 2 having anexcessively low molybdenum content, the welding joint had a poor degreeof tensile strength, 392.1 MPa, even though the low-temperature impacttoughness of the welding joint was high.

In the case of Comparative Sample 3 having an excessively high nitrogencontent, voids were observed in the welding joint after the weldingprocess.

Example 4

Flux-cored arc welding wires having a diameter of 1.2 mm were preparedas illustrated in Table 4 below. Cryogenic high-manganese steel having abasic composition of Mn₂₄C_(0.4)Cr₄Si_(0.3) was used as a base metal,and parts of the base metal were welded together using the flux-coredarc welding wires. The welding process was performed under conditions ofa 100% CO₂ protection gas, about 290 A DC, about 30 V DC, about 31 CPM,and a heat input of about 1.7 kJ/mm. In addition, an interlayertemperature was lower than 150° C., and a preheating temperature wasabout 100° C. to remove only moisture.

TABLE 4 Composition of wire (wt %) IP C Mn Si Cr P S N Mo B Ti Ni A1 A2A3 A4 A5 A6 W (J) TS IS1 0.1 34 1.2 3.5 0.01 0.005 0.26 3.5 0.01 0.5 9.5— — 13 0.01 1.7 1.5 Good 54 477.6 IS2 0.3 30 0.7 5.5 0.02 0.01 0.12 2.50.005 0.3 4.5 — — 9 1 1 1 Good 35 518.4 IS3 0.6 15 0.4 0.02 0.01 0.020.004 1.5 0.001 0.1 — — — 4 3 0.5 0.5 Good 31 427.48 IS4 0.1 30 0.7 20.01 0.01 0.09 2 0.005 0.1 5   4.5 — 15 3 1 0.6 Good 27 522.5 IS5 0.6 190.6 2 0.02 0.01 0.001 1.5 0.001 0.09 — 2.5 — 6 6 1 0.2 Good 29 480.92IS6 0.3 20 0.2 0.02 0.015 0.01 0.001 1.5 0.005 0.25 5.5 1.5 — 6 3 0.50.5 Good 32 448.62 IS7 0.3 20 0.6 1.2 0.015 0.01 0.13 2.5 0.005 0.25 5.5— 0.25 7 9 0.5 0.5 Good 41 452.9 CS1 0.3 20 0.6 6.5 0.015 0.01 0.04 2.50.005 0.1 5.5 — — 7 5 0.5 0.5 Good 14 567.7 CS2 0.6 20 0.6 1.9 0.01 0.010.082 2 0.02 0.6 — — — 8 5 0.5 0.5 Good 24 480.84 CS3 0.2 31 0.6 0.050.015 0.01 0.004 1.4 0.005 0.1 9   — — 7 5 0.5 0.5 Good 39 396.08 CS40.75 15 0.6 0.02 0.025 0.25 0.001 2 0.001 0.01 — — — 6 1 0.5 0.2 Poor(cracks) CS5 0.6 16 1.3 0.02 0.015 0.005 0.001 2 0.001 0.03 — — — 7 21.7 0.2 Poor (cracks) CS6 0.6 16 0.7 0.01 0.015 0.01 0.36 1.5 0.001 0.04— — — 8 4 0.5 0.4 Poor (voids) CS7 0.6 19 0.5 1.78 0.015 0.01 0.002 20.001 0.04 — — 1.25 7 2 0.5 0.2 Poor (voids) CS8 0.6 15 0.4 0.02 0.0150.01 0.004 1.5 0.001 0.1 — — — 17 3 0.01 0.5 Impossible CS9 0.6 15 0.75.5 0.01 0.02 0.001 1.5 0.001 0.1 — — — 6 10 0.01 0.5 Impossible CS100.3 30 0.4 0.02 0.015 0.01 0.12 2.5 0.005 0.3 4.5 — — 5 6 0.5 2Impossible IS: Inventive Sample, CS: Comparative Sample, W: weldability,IP: Impact Property, TS: Tensile Strength, A1: V + Nb + W, A2: Y + REM,A3: TiO₂, A4: SiO₂ + ZrO₂ + Al₂O₃, A5: K + Na + Li, and A6: F + Ca.

A Charpy impact test was performed (at −196° C.) to evaluate mechanicalproperties of welding joints formed through the welding process, andresults (J) of the test are shown in Table 4 above. In addition, thetensile strength (MPa) of the welding joints was measured as illustratedin Table 4 above. Those properties of the welding joints were measuredaccording to Korean Industrial Standards (KS), and the weldability ofthe welding joints was evaluated with the naked eye.

As illustrated in Table 4 above, in the case of Inventive Samples 1 to 7(flux-cored arc welding wires) having compositions proposed in theexemplary embodiment of the present disclosure, the welding joints had ahigh degree of weldability, a degree of low-temperature impact toughnesswith the range of 27 J or greater, and a degree of tensile strengthwithin a range of 400 MPa or greater. That is, the welding joints hadsatisfactory mechanical properties.

However, in the case of Comparative Sample 1 having an excessively highchromium content, the low-temperature impact toughness of the weldingjoint was poor, and in the case of Comparative Sample 2 having boron andtitanium contents outside the ranges proposed in the exemplaryembodiment of the present disclosure, the low-temperature impacttoughness of the welding joint was also poor. In the case of ComparativeSample 3 having a molybdenum content outside the range proposed in theexemplary embodiment of the present disclosure, the welding joint had apoor degree of tensile strength, 396.08 MPa, even though thelow-temperature impact toughness of the welding joint was high.

Furthermore, in the case of Comparative Samples 4 and 5 havingphosphorus and sulfur contents or silicon and titanium contents outsidethe ranges proposed in the exemplary embodiment of the presentdisclosure, cracks were observed in the welding joints after the weldingprocess, and in the case of Comparative Samples 6 and 7 having anitrogen or titanium outside the range proposed in the exemplaryembodiment of the present disclosure, voids were formed in the weldingjoints.

In the case of Comparative Samples 8 to 10 having a content of TiO₂,SiO₂+ZrO₂+Al₂O₃, K+Na+Li, or F+Ca outside the range proposed in theexemplary embodiment of the present disclosure, cracks were formed at ahigh temperature during the welding process, and thus welding wasimpossible.

While exemplary embodiments have been shown and described above, it willbe apparent to those skilled in the art that modifications andvariations could be made without departing from the scope of the presentinvention as defined by the appended claims.

1. A high-strength welding joint having a high degree of cryogenictoughness and obtained by welding parts of a cryogenic high-strengthhigh-manganese steel, the high-strength welding joint comprises, by wt%, carbon (C): 0.1% to 0.61%, silicon (Si): 0.23% to 1.0%, manganese(Mn): 14% to 35%, chromium (Cr): 6% or less, molybdenum (Mo): 1.45% to3.5%, sulfur (S): 0.02% or less, phosphorus (P): 0.02% or less, boron(B): 0.001% to 0.01%, titanium (Ti): 0.001% to 0.2%, nitrogen (N):0.001% to 0.3%, and a balance of iron (Fe) and inevitable impurities. 2.The high-strength welding joint of claim 1, further comprising at leastone selected from tungsten (W), niobium (Nb), and vanadium (V) in atotal amount of 5 wt % or less.
 3. The high-strength welding joint ofclaim 1, further comprising yttrium (Y) and/or a rare earth metal (REM)in an amount of 0.1 wt % or less.
 4. The high-strength welding joint ofclaim 1, further comprising nickel (Ni) in an amount of 10 wt % or less.5. The high-strength welding joint of claim 1, wherein thehigh-manganese steel has a basic composition of Mn₂₄C_(0.4)Cr₄Si_(0.3).6. A flux-cored arc welding wire having a high degree of strength and ahigh degree of cryogenic impact toughness, the flux-cored arc weldingwire comprises, wt %, carbon (C): 0.15% to 0.8%, silicon (Si): 0.2% to1.2%, manganese (Mn): 15% to 34%, chromium (Cr): 6% or less, molybdenum(Mo): 1.5% to 4%, sulfur (S): 0.02% or less, phosphorus (P): 0.02% orless, boron (B): 0.01% or less, titanium (Ti): 0.09% to 0.5%, nitrogen(N): 0.001% to 0.3%, TiO₂: 4% to 15%, at least one selected from SiO₂,ZrO₂, and Al₂O₃: 0.01% to 9% in total content, at least one selectedfrom potassium (K), sodium (Na), and lithium (Li): 0.5% to 1.7% in totalcontent, at least one of fluorine (F) and calcium (Ca): 0.2% to 1.5%,and a balance of iron (Fe) and inevitable impurities.
 7. The flux-coredarc welding wire of claim 6, further comprising at least one selectedfrom tungsten (W), niobium (Nb), and vanadium (V) in a total amount of 5wt % or less.
 8. The flux-cored arc welding wire of claim 6, furthercomprising yttrium (Y) and/or a rare earth metal (REM) in an amount of 1wt % or less.
 9. The flux-cored arc welding wire of claim 6, furthercomprising nickel (Ni) in an amount of 10 wt % or less.